Freeze drying has been proven useful in many fields, including food processing and laboratory analysis of organic materials. Freeze drying enables the removal through sublimation of solvents, including water, from a substance without destroying its cellular structure. Through sublimation, the substance being freeze dried remains in a frozen, solid form until it is dried, i.e., until all of the liquid is removed from that substance.
Sublimation occurs when the frozen substance is heat treated in a proper manner. If improperly treated, the frozen solvent within the substance melts rather than vaporizes, damaging the substance and often rendering it unusable. Accordingly, the temperature level within a flask typically used for freeze drying is critical to proper sublimation.
In a common freeze-drying operation, one end of a drying flask is secured to a manifold of a conventional freeze-drying apparatus, such as that shown and described in U.S. Pat. No. 4,017,983, issued to Douglas S. Fraser on Apr. 19, 1977, and entitled "Freeze Dryer." In another common operation, the drying flasks are placed in a so-called tray dryer. Typically, the volume of these flasks is between five (5) and one-hundred (100) milliliters.
In tray drying, the temperature of the substance within one drying flask on the tray is monitored by a thermocouple. To ensure proper temperature monitoring, the thermocouple should extend through the length of the substance and its end, the point of highest sensitivity, should be adjacent to but not contacting the bottom center of that flask. The thermocouple will then determine the temperature of the substance in the central lower portion of the flask.
The freeze drying of a substance occurs at the ice interface. It follows, therefore, that a substance contained in a flask will dry from the top downward and from the sides inward, thus leaving the bottom central section the last portion to dry. Since the drying of a substance is accompanied by a rise in temperature, it becomes essential to monitor temperature at this critical point so that one may control the freeze drying process accurately either by manual or automatic means.
Unless the thermocouple is placed at a correct location within that flask, and unless that thermocouple remains fixed at that location, incorrect temperature information can be transmitted to the operator or the electronic controls determining the process parameters. This in turn can lead to improper adjustments of temperature, and damage to or destruction of the substance being freeze dried.
One device for positioning a temperature sensor in a freeze drying flask is shown and described in U.S. Pat. No. 4,966,469 issued to Fraser at al. on Oct. 30, 1990. The device includes a stopper which is snap-fittingly secured to a flask. The stopper has an opening approximately in its center through which an annular tube extends into the flask. A thermocouple is coiled around and supported by the annular tube so that its free end is positioned in the center of the flask. However, such a device is not easily adjustable, making it difficult for use on flasks of various sizes. While it is possible to change the position of the free end of the thermocouple by changing the number of windings around the annular tube, it is a time-consuming process which stresses the thermocouple. Also, it is difficult to use such a device in smaller flasks because most of the thermocouple must be coiled around the annular tube. The present invention provides an easier way to adjust the position of a thermocouple, or similar probe, within any size flask without the need for coiling and uncoiling the thermocouple.